As a densitometer capable of measuring not only the density of a suspended material in a measuring fluid but also a material dissolved in a measuring fluid without involving the deposition of any suspended material on the inner wall of a density measuring tube, a type has been known which measures such a density with the use of a microwave. This type of densitometer is known in JPN PAT APPLN KOKAI PUBLICATION 59-19846.
FIG. 14 shows an arrangement of a microwave densitometer as disclosed in the PUBLICATION above. In this meter, a microwave of a frequency f1 from a microwave oscillator 70 is separated by a distribute device 71 into two waves, one of which enters a density measuring tube 73 via one wave guide 72 provided on a density measuring tube 73. This branched wave passes through the measuring tube 73 and enters one mixer 75 via the other wave guide 74 provided on the measuring tube 73.
The other branched wave enters the other mixer 77 via a phase shifter 76. A microwave of frequency f2 from oscillator 78 enters these mixers 75 and 77 via a distribute device 79. These frequencies f1 and f2 are mixed at the mixers 75 and 77 and a low frequency signal is taken out as a frequency f3=f1-f2 and input to a phase comparator 80. The phase comparator 80 detects a phase difference between a low frequency signal coming from the mixer 75 and that coming from the mixer 77.
If the phase delay of a microwave passing through a path B is given by .theta..sub.1 in a state in which a measuring fluid moves through the measuring tube 73 with a to-be-measured material not contained therein, then the phase delay of the phase shifter 76 is so set that the phase delay of the microwave passing through a path C coincides with .theta..sub.1.
If the phase difference between the phase of the microwave passing through the path B and that of the microwave passing through the path C is measured, by the comparator 80, in a state in which a measuring fluid moves with a to-be-measured material contained in the measuring tube 73, then the phase delay of the microwave passing through the path B is represented as a value proportional to the density of the to-be-measured material contained in the measuring fluid.
If the density is measured by detecting the phase delay of the microwave varying in accordance with the state of the density of the to-be-measured fluid, then there is a possibility that inconvenience as will be set out below will occur.
FIG. 15 shows a relation among a microwave (M1) before the phase is delayed by the phase shifter 76, a microwave (M2) phase-delayed with the phase shifter 76 and a microwave (M3) having a phase delay corresponding to the density of the measuring fluid passed through the path B.
The phase delay .theta..sub.2 of the microwave (M3) relative to the microwave (M1) becomes greater in accordance with an increase in the density of the to-be-measured material. In the case where the density of the material is at a high level, there is a possibility that, as shown in FIG. 16, the phase delay .theta..sub.2 will exceed 360.degree.. Even if the phase delay .theta..sub.2 exceeds 360.degree., an apparent (seeming) phase delay .theta..sub.2 ' lies between 0.degree. and 360.degree..
In the case where a true phase delay .theta..sub.2 (so called in distinct from an apparent phase delay .theta..sub.2 ') falls within an angle range of 0.degree..ltoreq..theta..sub.2 &lt;360.degree., it does not correspond to one rotation and that number of rotations, n, is called as n=0. In the case where the true phase delay .theta..sub.2 falls within an angle range of 360.degree..ltoreq..theta..sub.2 &lt;720.degree. after one rotation, that number of rotations, n, is called as n=1. In the case where the true phase delay .theta..sub.2 varies in an opposite direction (minus direction) and falls within an angle range of 0.degree.&lt;.theta..sub.2 .ltoreq.-360.degree., that number of rotations, n, is called as n=-1. The number of rotations, n, varies similarly.
Since, in the phase comparator 80, the true phase delay .theta..sub.2 is detected as an apparent phase delay .theta..sub.2 ', if the true phase delay .theta..sub.2 corresponding to the density of the to-be-measured fluid exceeds 360.degree., a result of measurement reveals apparently a low density in spite of being a high level. In spite of the true phase delay .theta..sub.2 being below 0.degree., on the other hand, a result of measurement reveals apparently a higher density than in actual practice.
With an increase in the transmission distance of a microwave in the measuring tube 73, the phase delay .theta..sub.2 becomes greater in accordance with the length of the transmission distance, presenting a similar problem as set out above.
When the density of the fluid is so measured by detecting the phase delay of the microwave, the problem occurs as set out above. JPN PAT APPLN KOKAI PUBLICATION 2-238348 discloses a method for measuring a density whereby one solution to the problem is made. According to the technique as disclosed in this Publication, it is possible to detect the density of a to-be-measured fluid by measuring, by means of frequency modulation, a speed variation occurring upon passage of a microwave through a to-be-measured fluid.